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GHSA-9qq8-cgcv-qmc9
No affected components available
Summary
An attacker can trigger an index out-of-bounds panic in Step CA by sending a crafted attestation key (AK) certificate with an empty Extended Key Usage (EKU) extension during TPM device attestation.
Details
When processing a device-attest-01 ACME challenge using TPM attestation, Step CA validates that the AK certificate contains the tcg-kp-AIKCertificate Extended Key Usage OID. During this validation, the EKU extension value is decoded from its ASN.1 representation and the first element is checked. A crafted certificate could include an EKU extension that decodes to an empty sequence, causing the code to panic when accessing the first element of the empty slice.
This vulnerability is only reachable when a device-attest-01 ACME challenge with TPM attestation is configured. Deployments not using TPM device attestation are not affected.
Mitigations
If you are unable to upgrade to v0.30.0 or newer, the attack can be mitigated by disabling or removing any ACME provisioners that use TPM device attestation (device-attest-01).
Fix
In v0.30.0, a bounds check was added to validateAKCertificateExtendedKeyUsage so that an empty EKU sequence is treated as a validation failure rather than causing a panic.
Acknowledgements
This issue was identified and reported by @1seal (Oleh Konko).
Embargo List
If your organization runs Step CA in production and would like advance, embargoed notification of future security updates, visit https://u.step.sm/disclosure to request inclusion on our embargo list.
Stay safe, and thank you for helping us keep the ecosystem secure.
The vulnerability can be exploited over the network without needing physical access. It is difficult for an attacker to exploit this vulnerability and may require special conditions. An attacker does not need any special privileges or access rights. No user interaction is needed for the attacker to exploit this vulnerability. The impact is confined to the system where the vulnerability exists. There is a low impact on the availability of the system.
Limited exploitation activity has been observed. Close monitoring and planned remediation are recommended.
The exploit probability is very low. The vulnerability is unlikely to be exploited in the next 30 days.
We did not find any exploit available. Neither in GitHub repositories nor in the Exploit-Database.
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